Standard Alloy 688 (nominal composition of 73.5% Cu, 22.7% Zn, 3.4% Al and 0.4% Co) is a well-known copper base alloy for such purposes as springs, switches, contacts, relays and drawn and fabricated parts. This particular alloy exhibits an excellent combination of high strength and good formability. One problem which can be attributed to Alloy 688 is lower stress corrosion and stress relaxation properties compared to other alloys utilized for the same purposes.
It has been known that the stress corrosion and stress relaxation properties of Alloy 688 and modified 688 could be significantly improved if the alloy were subjected to low temperature thermal treatments after cold working. To attain maximum advantage of increased stress corrosion and stress relaxation resistance properties, this particular heat treatment should exceed a temperature of 280.degree. C. This particular processing presents a further problem since Alloy 688 suffers from its disability to retain its full strength properties at temperatures higher than that necessary for the heat treatment. This disability is particularly evident at higher levels of cold deformation in processing this alloy. It can be seen that this problem makes it difficult or impossible to perform this particular type of heat treatment and also retain high strength in the cold deformed Alloy 688.
Accordingly, it is a principal object of the present invention to provide a method for increasing the high temperature thermal resistance of Alloy 688 and modified 688 without significantly affecting the strength properties of the alloys.
It is an additional object of the present invention to provide for such high temperature softening resistance as indicated above through the use of alloying additions rather than particular processing.
It is a further object of the present invention to alternately provide for improved stress corrosion and stress relaxation resistance in Alloy 688 and modified 688 by improving the high temperature softening resistance of the alloys.
Further objects and advantages of the present invention will become apparent from a consideration of the following specification.